Patent GB-957371-A (GB)
Title: Vapour-generating power plants
IPC: F22B29/12
Filed: 1962-07-27
Abstract: 957,371. Steam raising in forced flow boilers. SULZER FRERES S.A. July 27, 1962 [July 27, 1961], No. 29030/62. Heading F4A. In a forced flow, once through steam boiler operating at sub-critical or super-critical pressures means are provided whereby the boiler can be started without the need for by-passing the superheater or providing a cooling flow of working medium to the reheaters, the means consisting of a recirculation circuit whereby working medium is withdrawn downstream of the conversion zone and returned to the boiler upstream of this zone associated with a turbine by-pass conduit system designed for a flow of working medium not exceeding 30% of the full load flow of the boiler, the conduit system including in succession an expansion valve and a separator whence water passes to a condenser and steam passes to a deaerator and/or a feedwater preheater and excess flow valve to the condenser. In the form shown a pump 12 draws water from a storage tank 11 and forces it through a preheater 14 and control valve 15 into the boiler. The water flows through an economiser 16 and furnace wall tubes 19 wherein conversion occurs and steam flows through superheaters 20, 25 to a turbine stop valve 26. At normal load the steam flows through a high pressure turbine 27, reheaters 30, 31 and intermediate and low pressure turbines 34, 36 to a condenser 37. A pump 39 extracts water from the condenser and passes it through a heater 40 to a deaerator 41 whence it returns to the storage tanks. When starting the boiler and during low load operation working medium is withdrawn from the tube system downstream of the tubes 19 by a pump 22 and returned to the inlet of the tubes through a non-return valve 24 maintaing a safe velocity of medium in the tubes. Furthermore during starting the stop valve 26 is closed until the steam temperature has reached a level where water will not be formed in the turbine, the working medium during the starting period flowing through a turbine by-pass conduit system, the furnace being fired to provide a heat input which is less than 30% of full load firing. In the conduit system the medium is first expanded through a valve 42, to subcritical pressure when the boiler is operating at super-critical pressure, the steam-water mixture passing into a separator 44 in which a constant water level is maintained by a valve 49. Water flows from the separator to the condenser 37. Steam flows through a line 52 which divides into three branches, the first to the preheater 14 under control of a valve 56, the second to the deaerator 41 under control of a valve 55 which valves are respectively set according to pressure in the preheater and deaerator and the third to the condenser 37 under control of a valve 54 set according to pressure in the line 52, this branch also including a desuperheater 66. The three pressure sensing devices 61, 62, 64 respectively controlling the valves 57, 59, 56 are set so that with rising pressure the valve 55 opens first at a preset pressure admitting steam to the deaerator. Then this valve partially closes to maintain the deaerator pressure constant this action causing a signal to be transmitted to the valve 56 which opens to admit the excess steam to the preheater 14. The device 64 controls the valve 56 so as to maintain the preheater pressure constant. At a higher pressure the valve 54 opens to allow steam in excess of the deaerator and preheater requirements to flow to the condenser. When the live steam temperature is sufficiently high the turbine valve 26 is opened and the valve 42 is automatically set to provide only sufficient steam for deaerating and preheating, the excess flow valve 54 then closing. Finally the feed and firing rates are increased to the required load, the recirculation valve 21 is closed when the flow velocity in the furnace wall tubes is adequate and the by-pass valve 42 is closed, preheating and deaerating steam then being supplied by bleed lines, not shown, from the turbine.